Answer ( 1 )

The Little Boy bomb dropped on Hiroshima was equivalent to 16 kilotons of TNT, somewhat more than a third that of 1 gram of AM, and destroyed almost everything within 1.6 km of the blast while instantly killing everyone within ~3.5 km. The military estimated that similar effects could have been produced with only 2.1 kilotons of smaller, conventional bombs because much of the energy dissipated harmlessly into the atmosphere above the blast.
The most powerful nuclear weapon ever detonated by man (the Tsar Bomba, only as a test) had a yield of 50 megatons with a theoretical maximum of 100 megatons with a U-238 tamper, more than 1000 times that of one gram of AM, which destroyed wooden houses over 100 kilometers away and damaged windows 900 kilometers away but did not destroy humanity.
The asteroid that likely killed the dinosaurs had the yield of around 10 billion Little Boy bombs, so (if I have my math right) around 500 thousand Tsar Bombas, and excavated a crater 100 km wide and 30 km deep.

In summary, 1 gram of antimatter would destroy a city fairly easily, but you’d need a lot more antimatter to pose a threat to all of humanity (or cram all of humanity into a much smaller area)